During a golf swing, the golf shaft flexes or bows due to gravity and forces applied by the golfer to the club, especially longer clubs. During the initial phase of the downswing, the club shaft bows backwardly as the golfer's hands and grip end of the shaft accelerate, and the club is subject to centrifugal forces and bending moments. As the downswing continues, the center of gravity of the club head tries to catch up and align itself with the club shaft. Eventually the head accelerates past the nominal shaft axis so that at the point of impact the club shaft actually is bowed forwardly. To the extent this flexing is within the swing plane or hitting direction and creates forward deflection at impact, it has the desirable effect of increasing the “dynamic loft” of the club to cause a higher ball trajectory and imparting a greater force to the ball at impact.
“Dynamic loft” is the angle of the club face when it impacts the ball during the swing. In contrast, “grounded loft” or static loft is the loft angle of the club face when the clubhead is resting on the ground during set up. Thus, dynamic loft takes into account the static loft of the club, the attack angle of the swing, as well as the change in the orientation of the clubhead attributable to any bending of the shaft. For example, forward deflection of the shaft in the swing plane at the point of impact creates greater dynamic loft, causing a higher launch angle and ball trajectory for a given static loft of the club. With a traditional longer golf club and typical swing, the dynamic loft may be substantially greater than the club's static loft.
An additional dynamic effect occurring during a golf swing is shaft “droop.” During the downswing, the club head accelerates toward the ball. As this happens, the centrifugal force of the downswing applied to the moment arm between the shaft axis and the center of gravity of the club head creates a torque force which causes the shaft to flex and bow downward, out of the swing plane. The resulting “droop” of the clubhead creates a flattening of the lie angle of the golf club of up to several degrees depending upon the location of the clubhead center of gravity, the speed of the swing (and the resultant magnitude of the centrifugal force), the length and flexibility of the shaft, and other factors. Shaft droop tends to open the club face at impact, causing a ball trajectory tendency to the right, and decreases the dynamic loft, both of which are undesirable.
In general, at the point of impact, the magnitude of the droop deflection of the club shaft is larger than its deflection in the swing plane, especially near the head where the shaft diameter is the smallest. This is attributable to the fact that the moment arm (i.e., distance between the shaft axis and the clubhead center of gravity) in the droop plane is typically larger than the moment arm in the swing plane. Both droop deflection and swing plane deflection are greater and more of a factor with longer clubs such as drivers, fairway woods and long irons because the head is at the end of a longer moment arm. They also are more of a factor with graphite shafts which generally are more flexible than steel shafts.
There remains a need for a golf club shaft that mitigates the effects of droop and improves club performance, especially in the context of clubs having composite (i.e., graphite) shafts.